IANAL but I know my way around US federal firearms law. I am not aware of any law or rulemaking which would classify, for example, CAD files describing a lower receiver as an item subject to firearms regulations. There exist books sold openly and without restriction detailing how one can make improvised firearms, some of which if constructed would be illegal for possession by American civilians. I'm sure this information has by now made it to the Internet as well. Up to this point, such materials are treated as speech and thus strongly protected by the First Amendment. I see no reason why 3D printer instruction files would be treated any differently, under current law.

It's not hard to imagine a Congresscritter raising a fuss over concerns of "children printing machine guns" or some such nonsense, and thereby introducing legislation to regulate such materials, including executable scripts for 3D printing machines. It's even possible to imagine such legislation passing the House and Senate. Our current president would likely sign any such legislation he was given. However, it is very difficult to imagine the legislation surviving a Supreme Court challenge on First Amendment grounds.

Then again, I said the same thing about the McCain-Feingold Campaign Finance Reform Act, and SCOTUS upheld that 5-4, so what do I know.

Legally, the only difference between a CAD file of the AR-15 and an AR-15 repair manual should be that one is actionable by a machine. They're both a description of the operation of the object (and thus speech).

And let's go even further into the hypothetical future where we've made great advances in AI. Also, for whatever reason, a court has found weapons CAD files to be illegal. What will the law say when a machine is intelligent enough to decipher the same human-targeted AR-15 repair manual and reconstruct one from that information?